A6 C4 1.8 ADR Cooling system problem

Hello guys,
So i have a big problem wich allready cost me a lot of money, lol.
Cooling liquid started to boil, after few times of boiling, i had to remove cylinder head, replace gasket, skimming and so on. There was no signalization about boiling water and indicator was showing ~90C temp.
I have replaced thermostat and water pump. Had no effect, water was still boiling... Upper radiator hose was much hotter than lower, which was actually cold. Without thermostat- both hoses had the same temperature, but water was still boiing. After that i found some dirt between radiator and A/C condenser, cleaned everything. Replaced water temp sensors, found three of them. This time (without thermostat) water is not boiling anymore, but signalization and water lamp comes up while temperature indicator showing no more than 90C temp... And also, oil lamp also cames up, but after acceleration it goes off.
I think that i have to change radiatior, but what about sensors? Why there is 3 of them? One of them is grey and has 3 contacts, other two are the same: blue and has 2 contacts. And why there is cooling system signalization if water is not boiling and indicator shows no more than 90c?
By the way, if im driving approx 80km/h, temperature drops down to ~60C, signalization goes off (without thermostat). And signalization goes on again when im driving slowly and temperature indicator shows ~88C.
Any thought's?
Sorry for non ideal english, lol.

Does the system make pressure? The cap should make the system create 13-16 pounds of pressure in the cooling system to keep it from boiling. Raising the pressure raises the boiling point of water. Remember that as soon as water boils it can no longer absorb heat and the engine will rapidly start overheating. Also straight water will boil faster than a 50/50 mix of coolant and water. Try a new pressure cap on the reservoir.

Does the system make pressure? The cap should make the system create 13-16 pounds of pressure in the cooling system to keep it from boiling. Raising the pressure raises the boiling point of water. Remember that as soon as water boils it can no longer absorb heat and the engine will rapidly start overheating. Also straight water will boil faster than a 50/50 mix of coolant and water. Try a new pressure cap on the reservoir.

I forgot to mention that i have replaced the cap with new one. How to check the pressure? Upper and lower radiator hoses are quite soft... And also i think there is no problem with circulation, because i can see how water is coming back to reservuar from snall hose.

Well sounds like the radiator is bad. if the temp in and out of the radiator are the same temp, then theres no heat being exchanged. You have covered all other bases, radiator is the final part of the system.

Today I put new coolant G12 as required. Coolant is not boiling, but i have stange feeling that its running in higher temperatures than supose to be. Replaced new coolant temperature sensor with old one. While driving with old sensor, no signalization light comes on, indicator shows ~88C. With new sensor, signalization light comes on even before the cooling fan turns on and indicator still shows ~88C. Strange...
Also, I reduced a litlle bit of oil level in engine, because I though it was the reason why oil signalization light was coming on (level was litle higher than maximum), but that doesnt worked. It comes on on low reps and after acceleration goes off.
On monday i will try to change radiator, if i will be able to remove front bumper, lol. I can't find what is holding bumper from the sides. Remover two long bolt from forward of the bumper, but something is holing it from the sides.

I think you may have a partially clogged radiator. This would lead to showing low temps while coolant is boiling in the head and pressure is created. check for pressure in the system by squeezing the hoses: engine off, cold= no pressure, open cap to equalize what minimal pressure there is. engine started= no pressure or very little ( some hoses higher pressure than others?); Engine 90C= hoses hard to squeeze. top hose should still have liquid in it and bottom should be hot but less so than top hose. Feel the radiator for hot/cold spots.

One, thank you, i will try to do this test tomorow. Today I partially solved the oil light problem: i was using 5W30 synthetic oil, so i spilled out about 1 liter of that oil and added one liter of 10W40 semi-synth. Now the light came up only two times in 20km run, lol. So i think its also have relation with cooling problem, because oil gets hot and pressure drops down (indicator at panel shows no more than 80C!). Any thoughts about that?
And also, about coolant temperature sensor: even when my cooling liquid was boiling, indicator at panel was showing 90C, never exceeded that point. But with old sensor there was no light coming up. With new one- light commes up, but indicator showing no more 88C, as i mentioned before. By the way, the signalization light and temperature indicator sensor is the same (same module)! It's grey color sensor with 3 contacts. It makes me crazy, lol.
I hope that new radiator will solve those problems.

The oil light is coming on because the engine is running so hot, the oil is getting way thinned out from carrying all the temperature load from the engine since your radiator is clogged up. You will damage the engine continuing to run the engine like this. Thicker oil is a bandaid not the fix.

Prospeeder, oil temperature never exceeded 80C. And light goes off in 5-10sec after accelerating. Bad sensor?
Today i replaced radiator, coolant reservuar and put another new temperature sensor-switch (orinigal, german). Coolant not boiling, no cooling signalization ligt coming on. I think that old radiator was also working properly, because after cleaning edge between radiator and A/C condenser coolant was not boiling anymore, just that stupit chineese sensor-swith was giving signalization about high temp, but in the same time sending temperature signal not bigger than 90C, lol. So, I believe it was chineese sensor problem. Tomorow im gona make longer run to see if everything works properly with cooling system. I replaced EVERYTHING, some items replaced two or three times, lol.
But... The oil light still comes on, time after time. And goes off, as i sad, after 5-10secs.